Impaired driving and over 80 in Canada are covered by the same section of the Criminal Code. A person cannot be convicted of both offences in a single offence, however they are very different in how they are prosecuted.
Impaired driving is how alcohol affects you and your ability to drive – over 80 is the scientific measurement of alcohol in your blood.
A good way to think about this is a young driver who has never had a drink before may have one or two drinks and be under the legal limit yet be well impaired. A older driver, who has a very strong tolerance for alcohol, may be well over the legal limit but be very sober. Both are criminal in Canada and both can lead to similar consequences but you can only be convicted of one and not both.